Add unit tests for ListControlStringCollectionEditor

[Nora-Zhou01]

Related #10773

Proposed changes

• Add unit test ListControlStringCollectionEditorTests.cs for public properties and method of the ListControlStringCollectionEditor
• Enable nullability in ListControlStringCollectionEditorTests.cs

Microsoft Reviewers: Open in CodeFlow

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.74520%. Comparing base (e4085eb) to head (7a1c240).
Report is 352 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #12516         +/-   ##
===================================================
+ Coverage   75.01536%   75.74520%   +0.72983%     
===================================================
  Files           3047        3158        +111     
  Lines         631584      636134       +4550     
  Branches       46764       47003        +239     
===================================================
+ Hits          473785      481841       +8056     
+ Misses        154434      150829       -3605     
- Partials        3365        3464         +99     

Flag	Coverage Δ
Debug	75.74520% <100.00000%> (+0.72983%)	⬆️
integration	18.17498% <ø> (+0.24058%)	⬆️
production	49.31432% <ø> (+1.18968%)	⬆️
test	97.04561% <100.00000%> (+0.02913%)	⬆️
unit	46.50538% <ø> (+1.37924%)	⬆️

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